Fire alarm system using a plural oscillator radio transmitter



AHRQ; ITSM 4 Sheets-'Sheet 1 'ISS RETIRINCI P. M. BARTON 'ET Al.

June 20, 1961 FIRE ALARM SISTEM USING A PLURAL oscILLAToR RADIO TRANSMITTER Filed sept. 2o, 1956 l June 20, 1961 P, M, BARTON ETAL 2,989,621

FIRE ALARM SYSTEM USING A PLURAL OSCILLATOR RADIO TRANSMITTER Filed sept. 2o, 195e 4 sheets-sheet 2 I )I /6-2 65 I F YW/M June 20, 1961 P. M. BARTON Erm.

FIRE ALARM SYSTEM USING A PLURAL OSCILLATOR RADIO TRANSMITTER Filed Sept. 20, 1956 4 Sheets-Sheet 3 June 20, 1961 P. M. BARTON ETAL FIRE ALARM SYSTEM USING A PLURAL OSCILLATOR RADIO TRANSMITTER Filed Sept.- 20, 1956 4 Sheets-Sheet 4 United States Patent O 2 989,621 FIRE ALARM SYSTEM USING A PLURAL OSCILLATOR RADIO TRANSMITTER Paul M. Barton, Los Gatos, and Jo Emmett Jennings,

San Jose, Calif., assignors to Jennings Radio Manufactoring Corporation, San Jose, Calif., a corporation of California Filed Sept. 20, 1956, Ser. No. 611,079 7 Claims. (Cl. Z50-4) This invention relates to radio communication, and in particular to an improved emergency alarm system for reporting fires and the like.

It is common practice in cities and other densely populated areas to provide tire alarm boxes scattered throughout the area for transmitting an alarm to a lire station or other alarm receiving center. Heretofore, such ire alarm systems have generally been of a wired-telegraphic type that requires the installation of wires or cables connecting each alarm box to the receiving center. The present cost of installing such a iire alarm system is in the order of one thousand dollars per yalarm box, due largely to the expense of the connecting wires and cables. Consequently, the expense of installing an adequate wiredtelegraphic tire alarm system in new communities, or in expanding areas of older cities, can be a substantial nancial burden.

Furthermore, the connecting wires and cables in such a system are vulnerable to storm damage, and are especially vulnerable if located above ground. Even when the more expensive underground cable is used exclusively, the cable can be `damaged by lioods and other causes. As a result, the expense of maintaining the alarm system is considerable, and the reliability and utility of the system are materially reduced. In fact, in times of great emergency or catastrophe, such as wide-spread floods or severe storms, the wired-telegraphic alarm system is apt to be disabled at the very time when it is most urgently needed.

Accordingly, an object of this invention is to provide a practical, reliable and inexpensive tire alarrn system utilizing radio rather than wires for transmitting alarms from the alarm boxes to a receiving center.

In a practical iire alarm system utilizing radio transmission of signals from the alarm boxes, the transmitted radio signals must have unusual characteristics. Large areas will usually contain a number of unit alarm systems, each consisting of a receiving center and a cluster of nearby alarm boxes. The reliability of radio reception within a limited range, about one to three miles in a typical unit system, must be exceptionally high to insure that all alarms will be received at the nearest receiving center. On the other hand, it is not practical to assign a different clear-channel carrier frequency to every unit alarm system that may be installed. Consequently, a transmitted radio signal should never be received at distances substantially greater than the intended short range of one to three miles. Otherwise, an alarm in one unit system may cause interference and false alarms in other unit systems. Accordingly, another object of this invention is to provide a radio system having the aforesaid unusual transmission characteristics.

Since each alarm box is connected to its individual radio transmitter, an exceptionally compact, reliable and inexpensive radio transmitter is required. Another object of this invention is to provide a novel radio transmitter for emergency alarm systems.

For reducing interfering noise, which may be great during thunderstorms and the like, a narrow-band communication-type radio receiver preferably is employed at the receiving center. On Ithe other hand, the transmitters remain idle for long periods of time, and may change ice frequency to some extent. As a result, there may be a frequency misalinement between the receiver and transmitter that could prevent reception of an alarm. Another object of this invention is to provide a novel modulation system that materially reduces frequency misalinement diiiiculties, and at the same time permits the use of narrow-band receivers,

Briefly stated, in accordance with certain aspects of this invention whereby the foregoing and other objects and advantages are achieved, a unit re alarm system or the like consists of a receiving center and a plurality of alarm boxes situated Within a three lmile radius of the receiving center. Each alarm box is connected to its individual radio transmitter, and the alarm box contains a switch or other device that can be manipulated to turn on the transmitter for broadcasting a radio signal to the receiving center. The signals broadcast by different transmitters within the same alarm system are modulated with dilferent audio tones so that the alarm box from which the signal was sent can be identified. The receiving center is equipped with a radio receiver connected to iilter and alarm circuits that separate the different audio tones, indicate the receipt of an alarm, and identify the alarm box from which the alarm was transmitted.

Ordinary radio waves broadcast in the conventional manner do not have suitable transmission characteristics for a practical fire alarm system. According to the present invention, surface waves are employed at a selected frequency within a critical frequency range of 2250 kilocycles to 2700 kilocycles per second. As used herein, the term surface Waves refers to electro-magnetic waves traveling along and in close proximity to the earths surface, substantially confined to a region between a hundred or so feet above the earths surface and about fifty feet below the earths surface. Such waves are also commonly called ground waves, but the latter term is sometimes loosely used to include direct rays, such as microwaves in line-of-sight transmission, which is not contemplated in the present application wherein true surface waves traveling in contact with the earths surface are employed. Preferably the surface waves are launched by an underground directional antenna and are received by a nondirectional antenna close to the earths surface and connected to ground through the receiver coupling circuit.

A novel transmitter for the lire alarm system comprises -two crystal-controlled oscillators connected in parallel to the input of an amplifier. The two oscillators oscillate with a frequency difference equal to the frequency of an audio tone, and the oscillators `are coupled together so that the audio tone modulates the output signals of both oscillators to produce an unusual modulated signal that can be detected by the receiver despite a considerable frequency misalinement. This is an especially important feature in a lire alarm system, since the transmitters may remain idle for long periods of time without retuning, subject to wide changes in ambient conditions. Nevertheless, -a high reliability of reception must be maintained so that any alarm that may be sent will be received.

T he invention will be better understood from the following detailed descr-iption of an illustrative embodiment taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims. In the drawings:

FIG. l is a schematic diagram illustrating parts of a re alarm system embodying principles of this invention;

FIG. 2 is a circuit diagram of an alarm box and radio transmitter used in the system of FIG. l;

FIG. 3 is a partly schematic fragmentary circuit diagram of the receiver, lter and alarm circuits used at the receiving center of the system sho'wn in FIG. l; and

FIG. 4 is a fragmentary circuit diagram showing a i 3 transmitter modication for providing one-way voice communication.

Referring now to FIG. 1 of the drawings, a plurality of lire alarm boxes, such as boxes 1 and 2, are located within a radius of three miles from a receiving center equipped with a radio receiver 3. The receiving center may be at a fire station, a tire department message center, or any other central location where fire alarms will be received. The receiving center and the cluster of nearby alarm boxes from which it is to receive lire alarms is herein referred to as a unit alarm system. The tire alarm system for a large city may consist of many such unit alarm systems spread over the metropolitan area. If desired, other types of communications systems, such as microwave radio lrelay, may be employed to relay alarms from the unit alarm system receiving centers to an area or city-wide message center.

Each of the alarm boxes has a weatherproof housing similar in design and appearance to the housing of lire alarm boxes for wired-telegraphic iire alarm systems now in common use. For example, each alarm box may have a glass window 4 that can be broken for exposing a switch or other device to be manipulated for turning in an alarm. Or, the switch can be actuated automatically by the breaking of the glass. The alarm boxes can be mounted on conventional standards 5, or on telephone poles, or in any other convenient place.

Each alarm box is connected to its individual radio transmitter. A cable 6 connects alarm box 1 to a transmitter 7 having a loop antenna 8. Similarly, a cable 9 connects alarm box 2 to a transmitter 10 having a loop antenna 11. Preferably the transmitters and antennas are placed a few feet below the earths surface, in a manhole, sewer, or other underground chamber. For example, transmitter 7 is located in a concrete-walled manhole 12 covered by ya conventional iron manhole cover 13, and transmitter is located in a manhole 14 covered by manhole cover 15. Since nearby sewers, utility system manholes, and the like can usually be employed, it is only occasionally necessary to construct special manholes for the fire alarm system.

The antenna is housed in a waterproof plastic tube (a section of plastic garden hose has been used satisfactorily) and the transmitter is housed in a watertight metal box attached to one side of the antenna as shown. Thus the transmitter and antenna are protected from water and can operate during the most severe iioods. In fact, these transmitters have been operated successfully while submerged at the bottom of a salt-water bay under more than twenty feet of ocean water. Being underground and of small size, there is little likelihood that the transmitter and antenna will be damaged by any disaster or catastrophe, even including earthquakes. The parts most susceptible to damage are cable 6 and the alarm box itself, but damage to these above-ground parts is apparent and easily detected by casual inspection. There are no long and vulnerable wire transmission lines either under or above the earths surface.

At the receiving center, radio receiver 3 is connected through coupling circuit 16, which may be an ordinary coupling transformer, to an antenna 17 and a ground connection 18. The receiving antenna may be a vertical metal pole or whip extending upward a few feet above the earths surface. Receiver 3 may be a standard communications-type radio receiver. The output terminals of the radio receiver are connected to filter and alarm circuits 19.

The transmitters are normally oi or not transmitting, and are turned on when a switch in the 'alarm box is manipulated for sending in a tire alarm. All of the transmitters within a unit alarm system are operable on the same radio frequency channel, and receiver 3 is tuned to this same channel so that a signal transmitted by any transmitter within the unit system will be instantly received by the receiver without retuning. To identify '4 l the alarm box from which the signal was sent, the different transmitters within the unit `alarm system transmit signals that are modulated with different audio tones. Consequently, there is supplied at the output terminals of receiver 3 an audio tone having a frequency that identiiies the alarm box at which the alarm originated.

The filter and alarm circuits 19 contain a plurality of bandpass filters each Iadapted to transmit only the tone associated with a respective one of the alarm boxes, and the filtered signals control the operation of lamps or other indicator devices for displaying an identification of the box from which an 'alarm has been received. A substantial advantage of this system is that simultaneous reception of signals from different alarm boxes is possible, and the operation of one alarm box causes no interruption in the ability of other alarm boxes to transmit a signal that can be received at the receiving station.

The maximum number of alarm boxes that can be provided in each unit alarm system is limited to the maximum number of different separable and identifiable audio tones or tone combinations that can Ibe transmitted within a single radio-frequency channel. This in tum is limited by the frequency bandwidth of the radio channel, the resolution of the lter circuits (that is, the smallest frequency difference between two audio tones that the filters can separate) and the frequency stability of the system. A practical set of values is a frequency separation of cycles per second between audio tones (for example, audio tones of 400 cycles, 500 cycles, 600 cycles, etc.), a radio channel bandwidth of 10 kilocycles, a maximum modulation frequency of 4,000 cycles, and a minimum modulation frequency of 400 cycles. This makes available 37 different audio tones, or 37 alarm boxes per unit system.

Where more alarm boxes are needed in a given area, two or more unit alarm systems can be superimposed on the same tire protection area by providing at the receiving center two or more receivers tuned to receive signals from different groups of transmitters; or adjoining unit alarm systems can be made to overlap in area; or the area covered by each unit system can simply be reduced. Alternatively, a wider bandwidth for the radio frequency channel, or filter systems with higher resolution permitting closer spacing of the modulation frequencies, or more elaborate modulation systems such as modulation of each carrier with multiple tones, may be employed to provide more alarm boxes per unit system.

The lire alrm system in a large city may include many unit alram systems covering different areas of the city. It is evident that the same audio tone used to identify the alarm boxes in one unit system must also be used to identify the alarm boxes in other unit systems, and that an alarm transmitted from an alarm box in one unit system generally must not be received by the receiver in another unit system; otherwise, a false alarm would result. In the case of adjacent alarm systems each containing a small number of alarm boxes, it may be possible to assign different sets of audio tones to the two unit systems and in that way avoid interference between systems. Sometimes, reception of a transmitted signal by one receiver and not by another can be assured by appropriate orientation of the loop or other directional antennas. In other cases, interference between adjacent, superimposed or overlapping unit systems must be eliminated by assigning a different radio frequency channel to each unit system. By means of these techniques, interference between adjacent, overlapping and superimposed unit alarm systems can be avoided, usually without using more than three radio frequency channels.

It will be advantageous to secure the allocation of clear radio channels for lire alarm service, since other radio services using the same channels might cause interference and false alarms in the lire alarm system. Because of the great demand for radio channels, it is evident that a different frequency channel cannot be assigned to every unit .5 alarm system that may be installed. The allocation of three or four, or some other reasonably small number, of radio frequency channels to municipal fire alarm service, for the prevention of interference between adjacent, overlapping and superimposed unit alarm systems, is feasible and justifiable. The allocation of hundreds of radio frequency channels, for preventing interference between non-adjacent unit systems, is not justifiable. Therefore some other means must be utilized for preventing interference between non-adjacent unit alarm systems.

In other words, a radio signal transmitted upon operation of the switch in an alarm box must be capable of being received with high reliability within the short transmitter-to-receiver distances encountered within a single unit system, but must not be capable of reception by the receivers of similar systems located at substantially greater distances from the transmitter. Obviously, ordinary broadcasting techniques intended to transmit radio signals over relatively large distances are not suitable for use in an alarm system of the type herein described. In the present state of the art, microwave line-of-sight transmission is not suitable either, because of complexity, expensiveness, and unreliability of the equipment required. In fact, to the best of applicants knowledge, no radio system known prior to the present invention has the characteristics needed for a lire alarm system that is practicable and economically as well as technically feasible.

In accordance with the present invention, signals are transmitted from the radio transmitters to the radio receiver by means o-f electromagnetic surface waves traveling along and in close proximity to the earths surface. These surface waves are launched by a directional antenna having a large horizontal radiation lobe disposed near the earths surface.

Preferably a loop antenna positioned a few feet below the surface of the earth is employed, as hereinbefore explained. Although the azimuth directional characteristics of the underground loop are not as sharp as they would be for the same antenna located above ground, the underground loop s till has a substantial bi-directional radiation pattern in the horizontal plane, and a considerable gain in signal strength `over that available with a nondirectional antenna can be achieved by alining a major radiation lobe of the loop antenna with the azimuth direction in which the receiver is situated.

In addition to the use of an appropriate antenna for launching a surface wave, it is essential that the transmitted radio waves have a frequency within the critical frequency range of 2250 kilocycles per second to 2,700 kilocycles per second. Within this critical frequency range, and only within this frequency range, the surface wave attenuation rate is favorable and undesirable sky-wave components are almost completely suppressed.

For successful utilization in a fire alarm system of the type herein described, the surface waves as they travel along the earths surface must be attenuated at a rate that is neither too small nor too great. If the attenuation rate is too small, it is difficult to restrict the surface waves to the desired range, and interference may be produced in nearby unit alarm systems operating in the same radio frequency channel. On the other hand, if the attenuation rate is too great, an inexpensive low-power transmitter Will not provide signals of sufficient strength to be received reliably at transmission distances encountered in a unit system of convenient and economically feasible size.

In general, the attenuation rate increases with increases in frequency, being lowest for low radio frequencies and higher for higher radio frequencies. Within the frequency range utilized in practicing the present invention, a ten watt transmitter having a three-turn loop antenna four feet in diameter, hereinafter more fully described, provides signals that can be received reliably at distances up to about one mile. Greater distances, up to about three miles, can be achieved by reasonable increases in the transmitter power or the antenna size or both. On the .6 other hand, the surface wave attenuation rate is sufficiently great that intereference can easily be eliminated between nearby non-adjacent unit alarm systems operating in the same radio frequency channel simply by using lowpower transmitters having a maximum transmission range that is only moderately greater than the distance to the nearest receiver.

A more compelling reason why successful operation is limited to a small critical frequency range arises from the necessity for sky-wave suppression. Sky Waves are components of the electromagnetic energy radiated by the transmitting antenna that travel upward and are reflected back to the earths surface by reflecting layers in the upper atmosphere.

When the reiiected waves return to the earths surface near the transmitter, Where the surface-Wave component is strong, interference between the surface-wave and sky-wave components produces areas where the combined signal is very strong and other areas where the combined signal is very weak. A receiver located in one of the weak-signal areas may be unable to receive the transmitted signal even through the distance between the transmitter and the receiver is considerably smaller than the normal transmission range of the system.

Furthermore, as changes occur in the upper-atmosphere reiiecting layers, the weak-signal and strong-signal areas may shift in position relative to the earths surface, so that a receiver at a fixed location may at times receive strong signals and at other times receive no signal at all that can be detected and identified. Such changes may occur rapidly and frequently, and they are especially apt to occur at night when relatively low-level reflecting layers are present in the atmosphere. Consequently, the phenomenon is commonly known as night fading. Such fading, which produces unreliable reception, is intolerable in an alarm system that must operate with close to perfect reliability. rThe solution is to so suppress the sky wave that it is much weaker than the ground or surface wave within the normal operating range of the transmitter.

Other sky waves may return to the earths surface at distant points far beyond the range of surface-wave transmission, and may have substantial, easily-received signal strengths at distances many times greater than the surface-wave transmission range. Under certain conditions, even a ten-watt transmitter can produce sky waves that may be clearly received hundreds of miles away. Such long-distance sky-wave transmission could cause interference and false alarms in unit alarm systems at Substantial distances from the system Where the signal originated-in another part of a large city, or in another community, or even in another state. Such a situation is intolerable in a practical fire alarm system, and again the answer lies in the suppression of the sky wave.

Having determined that the sky wave must be suppressed, the question is how? Evidently the surface waves must be properly launched, with a major portion of the electromagnetic energy radiated by the antenna contained in a low-lying radiation lobe having a substantially horizontal axis. This alone is not adequate. Applicants have discovered that the frequency of the transmitted wave must lie within the critical frequency range 2,250 kilocycles to 2,700 kilocycles per second. Within this critical frequency range, and only within this frequency range, a satisfactory attenuation rate of the surface waves is obtained, and substantially complete suppression of the sky wave can be achieved.

At lower frequencies, the attenuation rate of the surface waves is too low for good control of the transmission range. There is a broad vertical radiation lobe from the transmitting antenna radiating sky waves that return to the earths surface near the transmitter and produce intolerable night fading of the received signal.

At higher frequencies, the horizontal radiation lobe tends to turn upward, so that the surface waves tend to 7 leave the earths surface and become sky waves. These sky waves can travel great distances before they return to the earths surface with relatively little attenuation, and they can interfere with and produce false alarms in alarm systems at substantial distances from the system where the alarm originates.

Within the critical frequency range herein disclosed, the attenuation rate of the ground waves is favorable for short-range transmission over distances up to three miles, and there is substantially complete suppression of the sky waves. For example, with a ten-Watt transmitter and a four-foot diameter underground loop antenna of the type herein described, applicants have found that a sky wave can be detected at an elevation of 500 feet above the earths surface only within a horizontal distance of 100 feet from the transmitter, and that even within this small area, almost directly over the transmitter, the received signal is very weak. On the other hand, using a slightly more powerful transmitter with a ground-level receiver located 21/2 miles away, test alarms were received with 100% reliability on an around-the-clock basis for a period of three months. During this period an alarm was transmitted at an average of once every three hours on a schedule that was not known to the personnel operating the receiver until after the transmission and reception records had been compared.

Reference is now made to FIG. 2, which is a circuit diagram of alarm box 1 and radio transmitter 7. The transmitter, which in the example given produces about 10 watts of output power, consists essentially of two crystal-controlled oscillators connected in parallel to the input of a linear amplifier that supplies RF power to loop antenna 8. The two oscillators in the embodiment illustrated include the two sections of a type 3A5 twin triode vacuum tube 20. Each section of this tube is an electron discharge device having an anode, a control grid and a filamentary cathode. One side of the filament of tube is connected to a grounded lead 21, and the other side of the filament is connected through a current-limiting ohm resistor 22 to a line 23 that may be connected to a 6 volt battery 24 by a switch in the alarm box when the transmitter is to be turned on, as is hereinafter explained.

Piezoelectric crystals 25 and 26 are connected between the anode and the control grid of respective sections of twin triode vacuum tube 20, as shown. The two crystals have different resonant frequencies, both of which lie within the critical frequency range of 2,250 to 2,700 kc. per second, and which differ from each other by an amount equal to the frequency of the audio tone that is to be used to identify the transmitter under consideration. For example, assuming that the transmitter is to broadcast at a carrier frequency of 2,450 kc. modulated with a 400 cycle audio tone, crystal 25 may have a resonant frequency of 2449.8 kc. while crystal 26 has a resonant frequency of 2450.2 kc. Two capacitors 27 and 28, having typical values of about 220 micromicrofarads each, are connected in series with crystals 25 and 26 to isolate the crystals from the D.C. anode voltage.

Two capacitors 29 and 30 in series are connected in parallel with crystal 25, and two capacitors 3-'1 and 32 in series are connected in parallel with crystal 26. A typical value for capacitors 29 through 32 is about 50 micromicrofarads each. The circuit junction between capacitors 29 and 30 and the circuit junction between capacitors 31 and 32 are connected to the filament of tube 20 through grounded lead 21. Grid leak resistors 33 and 34, having typical values of 100,000 ohms each, are connected between the two control grids of tube 20 and line 21. The two anodcs of tube 20 are connected through two load resistors 35 and 36, having typical values of 20,000 ohms each, to a line 37 that is maintained at a high positive D.C. potential, plus 315 volts for example, by a battery 38. Line 37 is maintained substantially at A.C. oround potential by a 0.01 microfarad bypass capacitor 39 connected between line 37 and a grounded lead.

The circuits including the two sections of twin triode vacuum tube 20 are two substantially conventional Piercetype crystal-controlled oscillators each operable to produce electric oscillations at substantially the resonant frequency of its piezoelectric crystal. However, the oscillator circuit is unique in that two anodes of tube 20 are coupled together by two capacitors 40 and 41 in series, having typical circuit values of about 25 micromicrofarads each, and the output line 42 of the oscillator is connected to the circuit junction between capacitors 40 and 41, as shown. As a result the signal generated by each oscillator is amplitude modulated with an audio tone having a frequency equal to the difference between the frequencies of the two oscillators, and the composite signal supplied through line 42, which is amplified and transmitted in a manner hereinafter explained, is modulated in an unusual manner and has novel characteristics that are highly advantageous for the type of radio signalling under consideration.

Except for the mutual coupling between the two oscillators provided by coupling capacitors 40 and 41, each oscillator operates in a conventional manner at a frequency that is closely regulated by its piezoelectric crystal. For example, considering the oscillator comprising the left section of tube 20, D.C. anode voltage is supplied from line 37 through resistor 35, and grid bias voltage is provided by the voltage drop due to grid current flowing through grid leak resistor 33. During positive peaks of the grid voltage, the control grid is slightly positive with respect to the cathode and the vacuum tube conducts a peak current that is almost directly proportional to the magnitude of the D.C. anode supply voltage and inversely proportional to the resistance of resistor 35. During the more negative portions of each alternating cycle of the grid voltage, the conduction of current by the tube is cut otf. As a result, the current conducted by the tube has an oscillation-frequency component with an amplitude that is a function of the magnitude of the D.C. supply voltage. The oscillationfrequency component of the anode voltage is likewise almost directly proportional to the magnitude of the supply voltage.

The feedback circuit between the anode and the control grid consists of crystal 25 and capacitors 27, 29 and 30. The capacitance of D.C. blocking capacitor 27 is so large that its A.C. impedance at the oscillation frequency is negligible. The total A.C. impedance of the electrical circuits connected to the anode is capacitive, and as a result the anode voltage has a lagging phase relation to the cathode-to-anode current through the vacuum tube.

At the oscillation frequency, crystal 25 has an inductive impedance so that the circuit consisting of crystal 25 and capacitor 30 in series supplies to the control grid an alternating voltage of proper amplitude and phase to maintain large-amplitude oscillations. As is well known, the oscillation frequency and amplitude stabilize at values such that the voltage gain around the feedback loop has a vectorial value of 1. Since both the magnitude and the phase angle of the crystal impedance change substantially with very small changes in frequency, the grid voltage is in proper phase to maintain the oscillations only at a particular frequency near the resonant frequency of the piezoelectric crystal, and consequently the oscillator always operates substantially at this frequency. The oscillator comprising the right section of vacuum tube 20 operates in the same manner. Since the two piezoelectric crystals 25 and 26 have slightly different resonant frequencies, the two oscillators operate at slightly different frequencies, both of which are accurately controlled by their respective piezoelectric crystals.

If there were no coupling between the two oscillators, each oscillator would produce at its anode an alternating voltage of substantially constant amplitude. However, because of the coupling provided by capacitors 40 and 41,

the output of each oscillator is amplitude modulated at a frequency equal to the difference between the two oscillator frequencies. This will now be explained.

Since the two oscillators operate at slightly diEerent frequencies, the phase relation between the oscillationfrequency components of their anode voltages changes constantly through an angle of 360 degrees. At times the two oscillators are momentarily operating in phase with each other, with a phase difference of zero degrees between their anode voltages. At other times the two oscillators are momentarily operating in phase opposition, with a phase difference of 180 degrees. In fact, the phase difference between the two sets of oscillations can be represented by a vector that rotates constantly through every phase angle between zero and 360 degrees at an angular velocity of 21r times the difference between the two oscillator frequencies.

Assume that the two oscillators are momentarily operating in phase with each other. As the instantaneous voltage drop across resistor 35 increases, the instantaneous voltage drop across resistor 36 increases by an equal amount and with the same polarity, so that there is substantially no voltage across coupling capacitors 40 and 41. Consequently, no current flows through the coupling capacitors, and each oscillator operates substantially at if the other were not present.

Now assume that the two oscillators are operating momentarily in phase opposition with each other. As the voltage drop across resistor 35 is increasing, the voltage drop across resistor 36 is decreasing, and there is a large alternating voltage across coupling capacitors 40 and 41 in series. Therefore current flows through the coupling capacitors between the anodes of the two oscillators. This current through the coupling capacitors has a frequency equal to the average of the two oscillator frequencies, and has an amplitude proportional to the sine of onehalf the phase angle between the two anode voltages.

Each oscillator acts as a synchronous rectifier that operates on the current flowing into its anode circuit through the coupling capacitors by rectifying the cornponent of such current that is in phase with the alternating anode-to-cathode current. For example, consider the alternating current flowing through coupling capacitors 40 and 41 into the anode circuit of the oscillator comprising the left section of tube 20. During the more positive portions of the control grid voltage when the left section of tube 20 is conducting current, the current passing through the coupling capacitors flows through the vacuum tube. During the more negative portions of the grid voltage cycle, when the left section of vacuum tube 20 is non-conductive, the current through coupling capacitors 40 and 41 must ow through resistor 35. Thus a rectified current flows through resistor 35, and this current has a D.C. component that is substantially proportional to one minus the cosine of the phase angle between the two anode voltages.

The D.C. component of current flowing through resistor 35 that is due to the current flowing through coupling capacitors 40 and 41 from the other oscillator, produces across resistor 35 a D.C. voltage drop that is equivalent to a reduction of the supply Voltage provided through line 37, insofar as the operation of the oscillator comprising the left section of vacuum tube 20 is concerned. Since the amplitude of the oscillations generated by the oscillator is a function of the magnitude of the supply voltage, the reduction in the effective magnitude of the supply voltage due to the aforesaid voltage drop in resistor 35 produces a corresponding reduction in the amplitude of the oscillations. Since the magnitude of the aforesaid voltage drop varies as a function of the phase angle between the two oscillatory anode voltages, the amplitude of the oscillations produced by the oscillator also vary as a function of the same phase angle. In other words the oscillation-frequency component of the voltages at the left anode of tube 20 are amplitude modulated at a frequency equal to the difference between the two oscillator frequencies. The oscillations produced by the oscillator comprising the right section of vacuum tube 20 are amplitude modulated in the same manner.

It should be noted that the output signal of each oscillator has its maximum amplitude substantially at the instant when both oscillators are momentarily operating in phase with each other, and has its minimum amplitude substantially at the instant when the two oscillators are operating momentarily in phase opposition with each other. The modulation index (that is, the ratio of the difference to the sum of the minimum and maximum amplitudes) depends upon the degree of coupling between the two oscillators.

If the impedance of coupling capacitors 40 and 41 is reduced relative to the impedance of resistors 35 and 36, as by making the coupling capacitors larger, the modulation index is increased. Conversely, a reduction in the capacitances of the coupling capacitors increases the reactance of the coupling circuit and reduces the modulation index. A modulation index of l, or percent modulation, is not feasible with the circuits illustrated, since this would correspond to a complete stoppage of oscillations at certain points in the modulation cycle, and erratic operation of the oscillators would result. However, any modulation index substantially smaller than one and greater than zero can be obtained by appropriate choice of the circuit parameters. The typical circuit values herein given as an illustrative example will provide a modulation index in the order of 0.4, which is a desirable value for reasons hereinafter explained.

The composite oscillator output signal supplied through line 42 is an alternating voltage having instantaneous values equal to the average of the instantaneous values of the two oscillator anode voltages. Its frequency is the average of the two oscillator frequencies, and the instantaneous amplitude of its modulation envelope is equal to the average of the instantaneous amplitude of the modulation envelope of the anode voltages times the cosine of one-half the phase angle between the two anode voltages.

Assuming that the modulation index for the amplitude modulation of each oscillator is in the order of 0.4, as hereinbefore explained, each half of the modulation envelope of the composite signal is substantially sinusoidal and similar to the modulation envelope representing 100 percent amplitude modulation with a sinusoidal modulating signal having a frequency equal to the difference between the two oscillator frequencies. However, the waveform of the composite signal differs from that of an ordinary amplitude-modulated signal in that the carrier of the composite signal undergoes a phase reversal at each minimum of the modulation envelope.

Consequently, the frequency spectrum of the composite signal supplied through line 42 differs greatly from that of an ordinary amplitude-modulated wave. In fact, the frequency spectrum of the composite signal is the sum of the frequency spectra of the two amplitudemodulated signals produced by the two oscillators. Since each oscillator produces `a signal having a frequency spectrum consisting of a carrier and two sidebands separated therefrom by frequencies equal to the difference between the two oscillator frequencies, and since one sideband in the frequency spectrum of each oscillator coincides with the carrier in the frequency spectrum of the other oscillator, the composite signal has a frequency spectrum consisting of four frequency components separated from one another by frequency differences equal to the difference between the two oscillator frequencies. The composite frequency spectrum is symmetrical about the mean frequency, and the two center frequency components are much larger than the two end frequency components.

Various modifications in the oscillator circuit can be made without departing from the broader inventive principles herein disclosed although the oscillator illustrated gives superior performance and is the preferred type at present. In the embodiment illustrated, each of the two crystal-controlled oscillators is of the Pierce type, but other types of oscillators having high frequency stability could be employed. For example, Miller-type oscillators having the piezoelectric crystals connected between the control grids and the cathodes may be used. Variations may also be made in the coupling circuit between the two oscillators. For example, a tapped inductor or the primary of a coupling transformer could be connected between the two anodes of vacuum tube 20 in place of capacitors 40 and 41, or a tapped resistor might be used, or more complex circuit networks could be employed.

However, the impedance of the coupling circuit should be of a similar nature to the net impedance of the entire oscillator anode circuit, so that the current through the coupling circuit will have the desired phase relation with the oscillator anode-to-cathode current. In other words, Pierce-type oscillators, such as the oscillators herein illustrated, have a capacitive anode load impedance, and therefore with this type of oscillator it is desirable to use a capacitive coupling circuit, which may conveniently be a pair of capacitors connected as shown. On the other hand, a Miller-type crystal-controlled oscillator, having the piezoelectric crystals connected between the control grids and the cathodes, has an anode circuit with an inductive impedance. Consequently, with Miller-type oscillators the coupling circuit would preferably be inductive, and might consist of a tapped inductor connected between the two anodes of tube 20. Other oscillators are designed to have a net anode load impedance that is essentially resistive, so that fthe anode voltage and the cathode-to-anode current are in phase and the Vacuum tube eliiciency is maximum. With such an oscillator, the coupling circuit may advantageously be a tapped resistor connected between the two vacuum tube anodes. In any case tuned circuits could be employed, with the tuning adjusted to give a capacitive, inductive, or resistive impedance, as required. When the coupling circuit is conductive with respect to direct current, as in the case of a tapped inductor, the two resistors 35 and 36 may be replaced by a single resistor connected between the center tap of the coupling circuit and line 37.

The composite oscillator output signal supplied through line 42 has instantaneous values that are substantially equal to the instantaneous average values of the A.C. voltage at the two anodes of vacuum tube 20. If the two oscillations at different frequencies were not amplitude modulated, the signal at line 42 would be the instantaneous sum of two sinusoidal waves of equal amplitudes and different frequencies, and would have a waveform that can be represented by the mathematical function E cos 1torneos wt, where w is equal to 211- times the average of the two oscillator frequencies, a is 2ntimes the difference between the two oscillator frequencies, and E is a constant. The modulation half-envelope or amplitude of this wave would have the form of a full-wave rectified sine wave, and the A.C. components of the rectified signal that would be obtained by passing such a wave through a conventional linear detector would be of the form In other words, such a signal would be amplitudemodulated with a frequency equal to the difference between the two oscillator frequencies, and its harmonics. It will be noted that the second harmonic would have an amplitude one-fifth as large as the fundamental. Any distortions in the transmission system or any nonlinearities in the detection device could increase the amplitudes of the harmonics relative to that of the fundamental. The large harmonic content or harmonic distortion might ring alarm circuits other than the alarm circuit tuned to 12 the fundamental modulation frequency, and thus produce false alarms.

In the actual case under consideration, the two oscillations are each amplitude modulated by their difference frequency, and the composite signal supplied through line 42 has a waveform that can be represented by the mathematical function E(1}m cos at) cos 1/zotLcos wt, where m represents the modulation index of the amplitude modulation occurring in each of the two oscillators. The waveform of the modulation envelope of this signal depends upon the value of m, but for all values of m greater than zero and not exceeding 1 the amount of harmonic distortion in the rectified or detected signal is less than it would be without the amplitude modulation. By a proper choice of m, any selected one of the harmonics can be eliminated. For example, for 111:7/19, there is no second harmonic component in the rectified signal. For m=?;/7, the amplitude of the largest harmonic is only about 2 percent of the fundamental component amplitude, and for practical signalling purposes the harmonic distortion can be considered negligible. In fact, any value of m between 0.3 and 0.5 gives a signal in which the largest harmonic is substantially smaller than 5 percent of the fundamental modulation frequency.

From the foregoing it is apparent that the twin oscillator herein described provides an exceptionally simple means for generating a radio-frequency signal modulated with a substantially pure audio tone. The simplicity of the apparatus, which -leads to reliability as well as low cost, is of vital importance in applications such as the radio fire alarm system.

The importance of low harmonic distortion is evident from the fact that different transmitters are identified by differences in their modulation frequencies or audio tones, and that as many different separable and recognizable audio tones as possible should be capable of utilization within a limited radio frequency bandwidth. Consequently, it will often be desirable to operate two different transmitters with harmonically-related modulation frequencies, such as one transmitter sending a signal modulated with and identified by an audio tone of 500 cycles per second, and another transmitter sending a signal modulated with and identified by an audio tone of 1,000 cycles per second. If the signal transmitted by the first-mentioned transmitter has a strong second-harmonic 1,000 cycle modulation component as well as its fundamental 500 cycle modulation component, confusion may result at the receiving station as to which transmitter sent the signal, or as to whether both or only one transmitter has been operated.

The importance of low harmonic distortion is even greater in the case of multiple-tone modulation of each carrier for increasing the permissible number of alarm boxes in a unit system. Multiple-tone modulation can be accomplished by amplitude-modulating the composite signal from the two coupled oscillators, or by coupling together more than two oscillators. For example, three oscillators operating at three different radio frequencies could have their anodes coupled together with a Y network.

The composite oscillator output signal supplied through line 42 has other unusual and advantageous characteristics. The radio frequency spectrum of this signal consists of four frequency components separated from one another by identical difference frequencies. For example, assume that the mean oscillator frequency is 2,450 kc., and that the difference between the two oscillator frequencies is 400 cycles per second. The frequency spectrum of the composite oscillator output signal consists of four frequency components having frequencies of 2449.4, 2449.8, 2450.2, and 2450.6 kilocycles per second, respectively. The two center frequency components (2449.8 and 2450.2 kc.) have substantially larger amplitudes than the end frequency components (2449.4 and 2450.6 kc.),

the amplitude ratio between the center and end frequency components being a function of the modulation index m, approximately An interesting characteristic of this frequency spectrum is that the audio tone or difference frequency of 400 cycles per second can be recovered by rectication or detection of all four frequency components, or of any two or three adjacent frequency components. This fact is of considerable importance in the tire alarm system, as will now be explained.

Since low-power transmitters are employed, it is desirable to use a high-gain sensitive receiver of the usual communications type for insuring reliable reception at all times. On the other hand, the system must operate reliably during thunderstorms and under other conditions where the radio noise level may be fairly high. As is well known, the signal-to-noise ratio, and therefore the reliability of reception under adverse conditions, is improved by using a highly-selective or narrow band receiver that is sharply tuned to receive the desired band of frequencies and no others. Consequently, the receiver bandwidth should be made as narrow as is practicable.

However, if a sharply tuned receiver is used and there is any substantial misalinement between the tuning of the transmitter and the tuning of the receiver, the transmitted signal may not be received at all. In a communication system where a signal is broadcast continually for long periods of time, the tuning of the receiver can be adjusted periodically or continuously to provide the best reception. In the fire alarm system, the tuning problem is much greater because the receiver should at all times be in tune with all of the transmitters in its unit system simultaneously, and furthermore a transmitter may not be operated to broadcast a signal for months at a time.

Even though the transmitter frequency is controlled by piezoelectric crystals, the frequency may change to some extent during the long periods between operation of the transmitter due to many factors, including aging of the crystal, substantial changes in anrbient temperature, and the like. Since the transmitters are to be operated from batteries, it is undesirable that the transmitters consume any electric power when they are not operating. Consequently, temperature controls for the crystals, such as thermostats and heaters, and other such methods frequently used in communication systems for stabilizing a broadcast frequency, are not feasible. Nevertheless, high reliability of reception is essential, and a signal must be received promptly whenever a transmitter is turned on.

Assume that a unit alarm system includes 37 transmitters operable at the same carrier frequency and identifiable by 37 different modulation frequencies for audio tones. Further assume that the 37 different modulation frequencies are separated from one another by frequency dfferences of 100 cycles, so that they can easily and reliably be separated and identified, that the lowest modulation frequency is 400 cycles per second, and that the highest modulation frequency is 4,000 cycles per second. To receive the signal modulated at 4,000 cycles per second, the radio receiver for this unit system must have a frequency bandwidth of at least 4,000 cycles. To provide a reasonable safety factor, assume that the actual receiver bandwidth is 6,000 cycles-that is, the frequency difference between the lowest radio frequency and the highest radio frequency that the receiver is tuned to receive is 6,000 cycles per second. If a larger bandwidth were used, the amount of noise received would be increased proportionately and the reliability of reception under adverse conditions would be reduced.

When all of the transmitters are accurately tuned to a carrier frequency, equal to the average frequency of the two oscillators in each transmitter, that is near the middle of 6,000 cycle wide frequency band that the receiver is tuned to receive, it is evident that a signal transmitted by any transmitter can be received by the receiver. In the case of signals transmitted with a low modulation frequency, all four frequency components in the radio frequency spectrum of the broadcast signal will lie within the received band, and there will be very little amplitude distortion in the detected signal. Consequently, there is little danger that the second harmonic of the 400 cycle signal, for example, will actuate the 800 cycle alarm circuit.

In the case of the higher modulation frequencies, such as the 4,000 cycle signal, only the two large-amplitude center frequency components of the radio frequency spectrum will be within the received band. However, upon detection or rectification of the received signal, the 4,000 cycle audio tone will be received together with several fairly prominent harmonics, as heretofore explained for two unmodulated signals of equal amplitudes that are added together and then rectified. However, there is no alarm circuit operating on a tone of 8,000 cycles per second or any other harmonic of 4,000 cycles, since it was assumed that 4,000 cycles is the highest modulation frequency used. Consequently, the harmonics in the rectied signal are of no consequence in this case.

Thus reliable detection -is obtained in all cases with a minimum receiver bandwidth, which increases the reliability of reception by reducing the received noise. It will be noted that the required receiver bandwidth is substantially less than would be received in the case of conventional double-sideband amplitude modulation, which for a 4,000 cycle tone would be 8,000 cycles per second. In this respect the system is comparable to single-sideband modulation, but utilizes much simpler equipment.

Now assume that there is some misalignment between the transmitter and receiver frequencies, as is quite likely to occur in a fire alarm system where the transmitters may not be operated for months at a time, and may be subject to considerable variation in ambient conditions. For example, assume that the nominal carrier frequency of the system is 2,450 kc., and that the receiver is correctly tuned to receive signals lying within the band between 2,447 and 2,453 kilocycles, but that one of the transmitters is off-frequency by about one-tenth of one percent and transmits a signal having a carrier frequency of 2447.5 kc. per second modulated with an audio tone of 4,000 cycles per second.

As hereinbefore explained, the transmitted signal has four frequency components symmetrically spaced about the carrier frequency in the frequency spectrum and separated from each other by frequency differences equal to the frequency of the modulating tone. In the example under consideration, the frequency components of the transmitted signal lie at 2441.5, 2445.5, 2449.5, and 2453.5 kc. per second. Only the two upper ones of these four frequency components lie within the reception band, but when detected these two components alone will yield the 4,000 cycle audio tone and will ring the proper alarm circuit. Consequently, the modulation system herein disclosed permits the reception of an intelligible signal even though there is a substantial frequency misalinement between the transmitter and the receiver. The amount of frequency drift stated in the above example is a maximum that may be expected in practice, provided crystals having stable characteristics are employed.

Similar considerations apply if the frequency misalinement is due to detuning of the receiver, or detuning of other circuits, such as the tuned antenna coupling circuits hereinafter described, that might cause attenuation of some of the four frequency components in a normal signal.

When both of the piezoelectric crystals 25 and 26 undergo similar small changes in their resonant frequencies, the only effect is a small drift in the carrier frequency, generally less than one-tenth of one percent, which is tolerable in the radio system herein disclosed. However, if the resonant frequencies of the two crystals change by different amounts, the modulation frequencies are changed, and the alarm may be incorrectly registered at the receiver. For this reason, the two piezoelectric crystals 25 and 26 should be carefully matched and selected to have identical frequency versus temperature characteristics. By careful selection of the crystal, adequate matching can be achieved.

The oscillator output signal supplied through line 42 is amplified by a substantially conventional linear amplifier, and then is supplied to antenna 8. The linear amplifier preferably includes a vacuum tube 43, which may be a type 5516 pentode having a filament connected between lines 21 and 23, as shown, and a control grid connected to line 42 through a 10 ohm resistor 44. The screen grid is maintained at a positive potential of 405 volts by a battery 45 connected to the screen grid through a line 46 and a filter circuit consisting of a 470 ohm resistor 47, a 0.005 microfarad capacitor 48, and a 0.01 microfarad capacitor 49. The suppressor grid of tube 43 is internally connected to the filament, and the anode is connected through a 2.5 millihenry inductor 50 to line 46.

The amplified signal is transmitted through a 0.002 microfarad coupling capacitor 51 to a tuned coupling circuit consisting of a 100 microfarad variable capacitor 52, a 350 micromicrofarad capacitor 53, a transformer having adjustably coupled windings 54 and 55, a 0.001 microfarad capacitor 56 and a 100 micromicrofarad variable capacitor 57 connected together and to antenna 8 in the manner shown in the drawing. Capacitor 57 is in series with antenna 8, and is adjusted for tuning the antenna circuit to resonance, whereupon radio-frequency voltage having amplitudes as high as 5,000 volts may be produced across capacitor 57. Therefore capacitor 57 is preferably a high-voltage vacuum capacitor.

The coupling between transformer windings 54 and 55 is adjusted to slightly greater than critical coupling, so that the passband of the coupling circuit is sufficiently wide for transmitting all four frequency components of the broadcast signal without substantially changing their relative amplitudes. Antenna 8 should be constructed to have very low electrical losses, and for this purpose preferably is made from three turns of No. 8 copper wire wound into a loop about four feet in diameter.

A small negative bias, about -45 volts for example, is supplied to the control grid of tube 43 by a battery 58 connected to line 42 through a l millihenry R.F. choke 59. The metal case of the transmitter, represented in FIG. 2 by a broken line, is connected to grounded lead 21 and forms a shield for the transmitter.

The metal housing of the alarm box, represented by a broken line in FIG. 2, is connected to ground as is indicated at 60, and is connected to the metal case of the transmitter by a lead 61 within cable 6 for grounding the case and line 21 of the transmitter. A jack 62 Within the alarm box is connected to line 46 of the transmitter through a lead 63 of cable 6, and a jack 64 in the alarm box is connected to battery 24 through a lead 65, to provide a convenient means for checking the battery voltages, whenever such may be desired, by means of conventional instruments connected in plugs that can be inserted in jacks 62 and 64.

The filament-supply line 23 of the transmitter is connected through a line 66 of cable 6 to a line 67 within the alarm box. Normally there is no current supplied to line 23, and consequently the filaments are cold and tubes 20 and 23 are nonconductive. When the two vacuum tubes are nonconductve. all of the circuits connected to batteries 38. 45` and 58 are open, so that there is no drain on the batteries when the transmitter is not operating. The transmitter is seldom operating. Consequently, the expected life of the batteries is essentially the same as their shelf life, and battery replacement is required only at infrequent intervals. When the transmitter is to be operated, connections between battery 24 and line 23 are completed within the alarm box, as hereinafter explained, whereupon current flows through the filaments and the vacuum tubes become conductive. The transmitter then operates to transmit a signal.

For system reliability, it is desirable that a transmitter operate for a certain minimum length of time whenever an alarm is to be sent in. This is necessary for several reasons: First, a short time interval is required to heat the vacuum tube filaments and to insure that the transmitter has reached a stable operating condition for transmitting a strong signal at approximately the correct frequency and modulation tone. Second, to insure reliable reception at the receiver despite the possibility of a high noise level, as when the alarm is sent during a thunderstorm or the like, and to insure reliable ringing of the alarm signal, it is desirable that the radio signal be received continually for a certain time interval.

On the other hand, it is undesirable that the signal be transmitted for an unnecessarily long time interval, for several reasons: First, unnecessary broadcasting serves no useful purpose, but simply runs down the transmitter batteries. Second, unnecessary transmission may interfere with the reception of alarms from other alarm boxes. Accordingly, the alarm box preferably contains circuits for operating the transmitter for a predetermined time interval each time that an alarm is sent in.

When an alarm is to be sent in, a push-button switch 68, or some equivalent device, is manipulated to close an electric circuit between a line 69 that is connected to battery 24 and line 67 that is connected to the vacuum tube filaments, as shown. As soon as line 67 is connected to battery 24, a relay 70 is energized, and the relay contacts 71 close to complete a circuit in parallel with switch 68 so that line 23 will continue to be connected to battery 24 after switch 68 is released. Current also flows through the heater 72 of a thermal switch 73, and after a certain time interval, depending upon the design or adjustment of the thermal switch, switch 73 opens and breaks the circuit through relay contacts 71.

Relay 70 then drops out, and the transmitter stops operating. The duration of transmitter operation can be adjusted by means of any adjustment mechanism provided in conjunction with the thermal switch, such as the adjustable resistor 74 connected in series with heater 72. An indicator lamp 75 may be provided for indicating when the transmitter is operating.

The simplicity and ruggedness of the transmitter is such that inspection and maintenance are required only at infrequent intervals. Present data indicate that a conservative maintenance program would be the following: check battery voltages and send in a test alarm once each six months; replace batteries when tests indicate weakness; and replace vacuum tubes every ten years.

Referring now to FIG. 3 of the drawings, the radio receiver 3 can be a conventional communication-type receiver that supplies detected audio tones in the form of audio-frequency voltages between output line 76 and grounded line 77. Lines 75 and 76 are connected to a plurality of bandpass filters, such as filters 78, 79, and 80. There is one such filter for each transmitter in the unit alarm system, and each filter is designed to transmit the modulation frequency of only a respective one of the transmitters. For example, filter 78 may have a passband of 350 to 450 cycles per second, so that filter 78 transmits the audio frequency signals provided between lines 76 and 77 only when a received signal is modulated with an audio tone of approximately 400 cycles per second. Similarly, filter 79 may transmit frequencies between 450 and 550 cycles per second, while filter 80 transmits frequencies between 550 and 650 cycles per second, etc. In this way the different modulation frequencies that identify different transmitters in the unit system are separated at the receiving station. Each of the filters transmits electric signals Within its passband to a different alarm circuit, such as the identical alarm cir- 17 cuits 81, 82, and 83. Since the alarm circuits may all be similar, only one is illustrated in detail.

Electric signals within the passband of filter 78 are transmitted to alarm circuit 81 as an audio-frequency alternating voltage between lines 84 and 85. This voltage is amplified by a conventional vacuum tube amplifier, which may consist of a type 6B6 pentode vacuum tube 86, a 400 ohm cathode resistor 87, a 20 microfarad cathode bypass capacitor 88, and the primary of a two-to-one stepdown coupling transformer 89, connected as shown. Anode and screen voltages are supplied by the secondary 90 of a transformer having a primary 91 connected to any suitable alternating current supply, represented in the drawing by plug 92, and a rectifier and filter circuit consisting of selenium rectifier 93, capacitors 94, 95, and 96, and resistors 97 and 98, arranged to supply a direct potential of about 100 volts to the anode and screen grid of tube 86.

A type 2D21 thyratron trigger tube has its control grid and cathode connected to a grid circuit consisting of the secondary of transformer 89, a type 1N34 crystal diode rectifier 100, a 2,000 ohm resistor 101, two 80,000 ohm resistors 102 and 103, a 2 microfarad capacitor 104, and a 0.005 microfarad capacitor 105. A negative bias of about 6 volts is supplied to the control grid by a bias circuit consisting of transformer secondary 106, selenium rectifier 107, 40 microfarad capacitor 108, and l megohm resistor 109, connected as shown. The shield grid of the thyratron is connected directly to its cathode, and is connected to its anode through a 0.1 microfarad capacitor 110. An alternating voltage of about 110 volts is supplied between the anode and the cathode of the thyratron by means of transformer secondary 90 and the circuit including a 1,000 ohm resistor 111 and the coil of a 50 milliampere D.C. relay 112 that is bypassed by a 20 microfarad capacitor 113.

Since alternating voltage is supplied between the anode and cathode of thyratron 99, the thyratron conducts during alternate half-cycles of the supply voltage whenever its control grid is insufficiently negative relative to its cathode for preventing such conduction. The thyratron is normally kept cut off by the negative bias voltage supplied to the control grid by the bias circuit including transformer secondary 106 and rectifier 107.

Now assume that a signal modulated with a 400 cycle audio tone is received by receiver 3, which supplies a 400 cycle per second alternating voltage between lines 76 and 77. This voltage is transmitted by bandpass filter 78 and amplified by the circuit including vacuum tube 86, so that an alternating voltage is produced across the secondary of transformer 89. Responsive to this alternating voltage across the transformer secondary, the circuit including rectifier 100 produces lacross capacitor 105 a voltage having a direct component that opposes the negative bias supplied to the control grid of the thyratron by the bias circuit. Consequently, the thyratron now conducts current during alternate half cycles of t-he alternating voltage across transformer secondary 90.

Since the conduction of current by thyratron 99 is substantially unidirectional, the thyratron current has direct and alternating components. The alternating components pass through bypass capacitor 113, but the direct components pass through the coil of relay 112 and actuate the relay. This closes the normally open contacts 114 of the relay, and operates an indicating device such as lamp 115. Since each transmitter in the unit system transmits a radio signal modulated with a different audio tone, and upon reception each audio tone passes through a different bandpass filter and results in the lighting of a different indicator lamp, the lamp that is lit identifies the transmitter and therefore the alarm box from which the alarm originated.

In addition to the indicator lamps, bells or other alarm devices can be provided and operated by other contacts on the alarm circuit relay for calling to the operators 18 attention the fact that an alarm is being received. If desired, latching type relays or other such means can be employed to keep the indicator lamp in a lit condition until the circuit is manually reset.

Earphones 116, or an amplifier and loud speaker, may be provided for aural monitoring of the received signals. Preferably, a peak clipping circuit is also provided for reducing noise by eliminating peaks or spikes in the received signal. The peak clipper may consist of a type 6H6 twin-diode vacuum tube 117, a pair of selenium rectifiers 118 and 119, two 40 microfarad capacitors 120 and 121, and a 5 volt center-tapped transformer secondary 122, connected as shown.

A further advantage of this alarm system is that it can easily be modified to provide one-way voice communication as well as an alarm signal, whenever such is desired. When this is done the versatility of the alarm is considerably increased, since it can then be used for civil defense and other purposes as well as for a fire alarm system. However, it is desirable that the alarm signal hereinbefore discussed be transmitted either simultaneously or in sequence with the voice communication, since this will always provide a sure identification of the alarm box from which the alarm originated, regardless whether or not the voice message is complete or even intelligible.

The carrier may be voice and tone modulated at the same time so that voice and tone signals are transmitted simultaneously, in which case additional filters may be employed at the receiver for filtering out the fixed-frequency tone signals before the voice signals are supplied to the earphones or loud-speaker. This additional complication can be avoided by sending the tone and voice signals sequentially, as will now be explained.

FIG. 4 is a fragmentary circuit diagram of a modified alarm box and transmitter having provisions for one-way voice communication as well as for fixed-frequency tonemodulated signalling. Most of the parts of the transmitter illustrated in FIG. 4 are identical to corresponding parts of the transmitter illustrated in FIG. 2, and therefore for clarity have been given the same reference numerals. However, in the FIG. 4 transmitter resistor 36 of one oscillator is not connected to line 37 directly, but instead is connected to a line 37' that is connected t0 line 37 through the normally closed contact 123 of a relay 124. Line 37 may be connected to grounded lead 21 through a bypass capacitor 39 to insure that line 37 is maintained at A.C. ground potential. The secondary of a modulation transformer is connected between bias battery 58 and choke coil 59, as shown. The primary 126 of the modulation transformer is connected to a microphone 127 located in or near the alarm box. A normally-closed contact 128 of relay 124 is connected in parallel with secondary 125, as shown.

When switch 68 in the alarm box is closed, the transmitter is placed in operation as `hereinbefore explained in connection with FIG. 2, and a tone-modulated signal is transmitted that identifies the alarm box from which the signal originates. A holding circuit similar to that shown in FIG. 2 may be provided in parallel with switch 68 so that the transmission will continue for a predetermined time interval even though switch 68 is released, as hereinbefore explained. At this time the secondary 125 of the modulation transformer is shorted by relay contact 128, and operation of the transmitter is identical to that hereinbefore explained in connection with FIG. 2. As soon as voltage is supplied to line 23 by the closing of switch 68 to place the transmitter in operation, current begins to flow through the heater 129 of a normally-open thermal switch 130. After a time interval determined by the setting of adjustable resistor 131 connected in series with heater 129, switch 130 closes and connects the coil of relay 124 between line 23 and grounded line 21, so that relay 124 is energized. This opens normally-closed relay contacts 123 and 128. At the same time, operating volt- 19 age is supplied to the microphone circuit through the primary 126 of the modulation transformer, and an indicator lamp 132 in the alarm box lights to indicate that the system is now in condition for transmitting a voice message.

As soon as relay contact 123 opens, the anode voltage supplied to one of the oscillators is interrupted, and one oscillator stops operating. Now the signal transmitted through oscillator output line 42 is a substantially unmodulated single-frequency electric signal that is amplified by the circuit comprising tube 43 and transmitted by the antenna connected to the transmitter. Whatever sounds are received by microphone 127 are converted into electric signals that are supplied through the modulation transformer 12S-126 to the control grid of vacuum tube 43, which now acts as a conventional grid modulator so that the radio-frequency signal supplied by the oscillator is amplitude modulated by the audio-frequency signal supplied by microphone 127. Thus a voice-modulating carrier is produced that can be received, demodulated, and reconverted to sound waves by a conventional radio receiver.

There are several advantages in the sequential system for tone and voice transmission. In the first place, the tone signal positively identities the alarm box from which the signal originated, regardless of what the voice message may be. In the second place, the preliminary interval before voice transmission can start gives the transmitter time to warm up and begin operating reliably on its assigned frequency, so that there is less likelihood that part of the voice message may be lost.

It should be understood that this invention in its broader aspects is not limited to the specific embodiments herein illustrated and described, and that the appended claims are intended to cover all changes and modifications that do not depart from the true spirit and scope of the invention.

What is claimed is:

l. A radio transmitter comprising two electric oscillators that oscillate at different frequencies, each of said oscillators having a terminal at which it provides an oscillation-frequency alternating electric signal having an amplitude that varies as a function of the mean D.C. potential of such terminal and at which applied alternating electric signals are rectified, a D.C. voltage supply, circuit means including a substantial resistance connecting each of said terminals to said voltage supply, a coupling circuit connected between said terminals permitting the ow of alternating current therebetween, said current being synchronously rectified by said oscillators and producing in said resistance a voltage drop substantially proportional to the sine of the instantaneous phase angle between said oscillatory signals whereby each of said signals is amplitude modulated with a sinusoidal wave having a frequency equal to the difference between said oscillation frequencies, means for adding portions of said two amplitude modulated signals to form a complexly modulated composite signal, and means for transmitting said signal in the form of radio waves.

2. A radio transmitter comprising two electron discharge devices each having an anode, a cathode and a control grid, a D.C. voltage supply, circuit means including a substantial resistance connecting each of said anodes to said voltage supply, feedback circuits connected to the electrodes of each of said devices such that oscillatory electric signals of different frequencies are provided at respective ones of said anodes, and a coupling circuit connected between said anodes and permitting the flow of alternating current therebetween, said current being synchronously rectied by said discharge devices and producing in said resistance a voltage drop substantially proportional to the sine of the instantaneous phase angle between said oscillatory signals, whereby each of said signals is amplitudey modulated with the difference 20 l between said frequencies, means for adding portions of said two amplitude-modulated signals to form a complexly modulated composite signal, and means for transmitting said composite signal in the form of radio waves.

3. Apparatus for generating amplitude-modulated alternating electric signals, comprising two vacuum tube triodes each having an anode a control grid and a cathode, a voltage supply, circuit means including a substantial resistance connecting each of said anodes to said voltage supply, two feedback circuits connected between the electrodes of respective ones of said triodes for producing two oscillatory signals at respective ones of said anodes, each of said feedback circuits including a piezoelectric crystal for stabilizing the oscillatory frequency of one of said signals, said two crystals having different resonant frequencies so that said two signals have different frequencies, and a coupling circuit connected between said two anodes for permitting the ow of alternating current therebetween, the impedance of said feedback circuit being similar in nature to the net impedance of all circuits connected to each of said anodes, whereby each of said oscillatory signals is amplitude-modulated with a frequency equal to the difference between said two oscillatory frequencies, the modulation envelopes of said two oscillatory signals having maximum amplitudes substantially at the instant when the two oscillatory signals are momentarily in phase and having minimum amplitudes substantially at the instant when said two oscillatory signals are momentarily in phase opposition.

4. Apparatus for generating amplitude-modulated alternating electric signals, comprising two vacuum tube triodes each having an anode, a control grid and a cathode, -voltage supply means providing a positive D C. potential relative to said cathodes, two resistors connecting respective ones of said anodes to said voltage supply means, two grid-leak resistors connecting respective ones of said control grids to said cathodes, two feedback circuits connected between the electrodes of respective ones of said triodes, each of said feedback circuits including a piezoelectric crystal between the control grid and the anode, a capacitance between the control grid and the cathode, and a capacitance between the anode and the cathode, whereby oscillatory electric signals having frequencies controlled by said piezoelectric crystals are produced at respective ones of said anodes, said two piezoelectric crystals having different resonant frequencies to that said two oscillatory signals have different frequencies, and two capacitors connected in series between said two anodes forming a coupling network permitting the ow of alternating current from one anode to the other, whereby each of said oscillatory signals is amplitudemodulated with a frequency equal to the difference between the two oscillation frequencies, and an output line connected to the circuit junction between said two coupling capacitors for transmitting a signal having instantaneous values proportional to the sum of the instantaneous values of said two amplitude-modulated signals.

5. A transmitter for radio fire alarm systems, cornprising two vacuum tube oscillators operable at different frequencies within the frequency range of 2250 to Z700 kilocycles per second, the difference between said two frequencies being equal to the frequency of an audio tone, a coupling circuit connected between said two oscillators such that each produces an oscillatory electric signal that is amplitude modulated with a substantially sinusoidal modulation envelope having a frequency equal to said difference frequency and a modulation index between three-tenths and tive-tenths, an oscillator output line con nected to the center of said coupling circuit for transmitting -a signal proportional to the sum of said two amplitude-modulated oscillatory signals, a vacuum tube amplier operable to amplify said composite signal, a loop antenna for transmitting the amplified signal, batteries for supplying anode -voltage to said vacuum tube oscillators and said vacuum tube amplifier, a battery for supplying filament current to said vacuum tube oscillators and said vacuum tube amplifiers, an alarm box containing a normally open switch connected in series with the lastmentioned battery so that the transmitter is normally off, whereby upon the closing of said switch the transmitter operates to broadcast a radio-frequency signal that is modulated with an audio-frequency tone.

6. A fire alarm system comprising a radio receiver receptive to electromagnetic surface waves having a selected carrier frequency Within the frequency range of 2250 to 2700 kilocycles per second, a plurality of underground radio transmitters located within a three-mile radius of said receiver, a plurality of lire alarm boxes each containing a switching device connected by wires to arespective one of said radio transmitters, each of said transmitters being normally off and being operable by manipulation of the switching device to which it is connected for transmitting short-range electromagnetic surface waves substantially at said selected carrier frequency, each of said transmitters comprising two oscillators that oscillate at different frequencies for producing two radiofrequency signals having a mean frequency substantially equal to said selected carrier frequency and a difference frequency within the audio range, said mean frequency being substantially the same in all of said transmitters and said difference frequency being different in different ones of said transmitters, and a coupling circuit connected between said oscillators for amplitude modulating each of said signals with a substantially sinusoidal modulation envelope having an audio frequency equal to the dilference between said two radio frequencies.

7. In a iire alarm system, an alarm-transmitting station comprising an alarm switch, a microphone, two normally off oscillators that operate at different frequencies when turned on for producing two alternating electric signals having different radio frequencies, a coupling circuit connected between said oscillators for amplitude modulating each of said signals with a substantially sinusoidal modulation envelope having an audio frequency equal to the difference between said two radio frequencies, connections for turning on both of said oscillators simultaneously upon manipulation of said switch, whereby a station-identifying, tone-modulated signal is transmitted, a timing device for turning oi only one of said oscillators after the tone-modulated signal has been transmitted for a fixed time interval, and means connecting said microphone to modulate the signal provided by the oscillator that remains on.

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